1. Field of the Invention
The present invention relates to a catalyst for purifying the contents of exhaust gas from an internal combustion engine, and metal oxide particles suitable as a carrier for an exhaust gas purifying catalyst and a method for preparing this.
2. Description of the Related Art
Exhaust gas from an internal combusting engine such as an automobile engine comprises nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbon (HC) and etc. These contents of the exhaust gas can be purified by the use of an exhaust gas purifying catalyst which oxidizes CO and HC while reduce NOx. A representative exhaust gas purifying catalyst includes a three way catalyst which comprises porous metal oxide carrier such as γ-alumina and a noble metal such as platinum (Pt), Rhodium (Rh) and/or Palladium (Pd) carried thereon.
It is necessary that an internal combustion engine is driven at stoichiometric air/fuel ratio (stoichiometry) in order for the three way catalyst to achieve effective oxidation of CO and HC as well as reduction of NOx. In the case that the internal combustion engine is driven at excess oxygen atmosphere (lean) or at excess fuel atmosphere (rich), the three-way catalyst cannot demonstrate its purification ability as the oxygen concentration in the exhaust gas becomes outside the range of the optimum oxygen concentration for the three-way catalyst.
It is well known that a material having an oxygen storage capacity (OSC) is used with an exhaust gas purifying catalyst. OSC means a capacity enabling storing oxygen at the high oxygen concentration and releasing oxygen at the low oxygen concentration. OSC is useful to buffer the change of oxygen concentration in exhaust gas and thereby enhances the exhaust gas purifying ability of the three way catalyst. A representative material having OSC is ceria (CeO2). Ceria has not only OSC but also large affinity with noble metal carried thereon. Therefore, the ceria is also useful to prevent particle growth (sintering) of the noble metal carried thereon. Methods for preparing mixed metal oxide of ceria and zirconia have been developed to provide materials having high heat resistivity as ceria has small specific surface area and low heat resistivity. Regarding the prior arts, refer to Japanese Unexamined Patent Publication No. 8-103650, 8-109020, 8-109021, 2000-319019, 2001-89143 and etc.
According to the prior arts, as both ceria and zirconia exist on the surface of the mixed metal oxide comprising uniformly mixed ceria and zirconia, a noble metal carried by the mixed metal oxide randomly deposits on both ceria and zirconia surface. Therefore, in the prior arts, there is a problem that an affinity between the mixed metal oxide and noble metal is lowered, the noble metal is sintered and, then, the catalyst loses its ability to purify exhaust gas.
That is, the previous catalysts comprising a cerium-zirconium mixed metal oxide lose OSC, and an ability to purify exhaust gas, by the sintering of noble metal on their surface, particularly when they are exposed to a high temperature of 1000° C. or more for a long time.
Therefore, there remains a need for an exhaust gas purifying catalyst which maintains a heat resistivity of mixed metal oxide, has high affinity to a noble metal to maintain OSC, and only slightly lose its ability to purify exhaust gas after being exposed to a high temperature. Further, there remains a need for a method for preparing a metal oxide suitable for the exhaust gas purifying catalyst.